But human faces are usually not symmetrical. There was once a photographic study taking full front, evenly lit portraits of non-celebrity subjects. The faces were divided down the middle. Each half was joined to a mirror image of itself. Some would recognizably be the subject, but the other mirrored half was like a Jekyll/Hyde thing.

You've missed the point entirely. Real life asymmetries are pretty subtle. You usually won't find a person whose eyes are not essentially on the same level. The gross anatomy is pretty symmetrical but the details are where the variance is. Mostly in ears and the angle of the nose... but it's all usually subtle and when it isn't, it's really noticeable.